This invention relates to a thin film magnetic disk recording medium. More particularly the invention relates to texturing a recording medium.
Generally, magnetic thin film disks comprise a substrate on which a non-magnetic underlayer, a magnetic layer and a protective overcoat are deposited by vacuum deposition, followed by application of a thin layer of lubricant. An example of such a disk is described in U.S. Pat. No. 5,180,640, incorporated herein by reference. It is known to use, as a substrate, ceramic material, glass, or an aluminum alloy coated with electrochemically deposited nickel-phosphorus ("NiP").
During use, the disk is rotated rapidly by a motor. In order to read and write data on the disk, a recording head "flies" very close to the rotating disk. The recording head typically rests in contact with the disk when the disk is not rotating. To prevent severe adhesion (stiction) between the head and disk, the surface of the disk is intentionally roughened or "textured". (The problem of stiction is described in U.S. Pat. No. 5,053,250, column 1.) For magnetic disks comprising metal substrates coated with NiP, texturing is usually done by abrading the electrochemically deposited NiP surface with fine particles to form circumferential ridges. Other texturing methods are also known in the art. For nonmetallic substrates, such as glass, texturing may be done by chemically etching the substrate to leave bumps, or covering the surface with fine particles. U.S. Pat. No. 4,833,001 (incorporated by reference) discloses a typical etching process.
To maximize the recording density, the recording head flies as close to the disk surface as possible. Hence the roughness of the disk surface is minimized, consistent with maintaining sufficiently low stiction.
The abrasive texturing method cannot be used for typical non-metallic disk substrates because of their hardness, so other methods such as chemical or plasma etching or coating the substrate with a fine dispersion of particles, have been adopted. However, these processes, like abrasive texturing, are expensive in production as they require a number of process steps to be applied to each substrate. Additionally, in the case of texturing NiP, any irregularities that protrude sufficiently above the normal textured surface can damage the head while the head is flying and can cause the head to crash. Also, each texturing method is specific to a particular substrate material so that changing the composition or type of the substrate material requires the development of a new texturing method.
European Patent Application No. 0 399 747 A, published Nov. 28, 1990, advocates texturing a substrate by forming a discontinuous metal layer over the substrate surface. This layer forms a discontinuous multiple island structure over the plane of the substrate surface. The '747 method depends critically on the wetting between the islands and the substrate which restricts the choice of materials and makes the substrate cleanliness and absence of surface contaminants critical. The islands are coated with an "oxygen trap" layer of Ti, and then a thick Cr underlayer/magnetic layer/overcoat stack. The Ti layer contacts the substrate and bonds the islands to the substrate since the islands themselves poorly adhere to the substrate. Of importance, the '747 patent application requires formation of an island structure rather than a continuous layer, or deterioration occurs at the interface between the metal used to form the islands and underlying substrate. See, e.g., the "comparative example" at '747 column 8.
Another island-forming layer is discussed in U.S. Pat. No. 5,053,250. Drops of Ga or other transient liquid metals (e.g. In, Sn, Bi, Pb or other metals) are coated directly on and react with the magnetic film. The disk texture results from the reacted layer. Hence the Ga layer determines both the texture and magnetic characteristics, which makes it difficult to control both independently.
In U.S. Pat. No. 5,082,709 particle-like projections are formed in circumferential bands on a smooth or initially circumferentially textured substrate, and thereafter a magnetic layer is applied to the substrate. The projections are arranged in bands to enhance magnetic anisotropy. The '709 patent states that the projections are fine particles with heights preferably of 10 to 50 nm and an areal density of 10 to 1000 .mu.m.sup.-2, and cover between 10 and 80% of the disk. According to the '709 patent, if they cover too much of the disk stiction is too high. The '709 patent mentions forming the projections by several techniques, including sputtering, ion beam deposition or by putting particles in a binder and brushing the particles and binder onto the substrate. For particles in a binder, the projections are arranged in bands by circumferential brushing. For projections formed by the other above-mentioned techniques, it is not entirely clear how the projections are formed into bands, or how coverage of 10 to 80% of the disk is obtained.